Lymphocyte numbers are regulated both by responses to conventional exogenous antigens and by stimulation by endogenous peptide/MHC complexes. This joint regulation permits individuals to maintain a broad repertoire of specificities, allowing responses against a vast array of foreign substances and, at the same time, providing a pattern of memory based on the immunization history of the individual. The study of the process of lymphocyte dynamics that underlies this regulation requires a multidisciplinary approach, aimed both at the molecular underpinnings of the processes through which lymphocytes survive and proliferate and a systemics/ computational biology approach to appreciate the overall mechanisms governing total numbers of lymphocytes of distinct phenotype and distinct specificity. Emphasis has been placed on four aspects of this problem: the priming of naive CD4 T cells, the dynamics of lymphocyte memory, the mechanisms underlying CD4 T cell depletion in HIV infection, and the process of homeostatic proliferation. [unreadable] It has been shown that primary responses are highly dependent upon the number of precursor cells that can respond to antigenic challenge. Using both real time PCR and flow cytometric analysis to measure the response when TCR transgenic cells are transferred to intact recipients, it has been shown that the factor of expansion (FE) of TCR transgenic T cells transferred to syngeneic hosts is highly dependent upon the number of precursors. When the frequency of precursors in the recipient is 3 or less, the FE is 1500, at 300 it is 200 and at 30,000 it is 20. The limitation in expansion does not result from a smaller fraction of cells responding but rather, at least in part, from diminished proliferative rates of responding cells. The diminution in FE as the number of precursors increases cannot be accounted for by Fas-mediated cell death nor can it be due to limitation in numbers of dendritic cells or in amounts of antigen as increasing either DC number or amount of antigen does not alter the non-linearity of FE and precursor number. Furthermore, the effect is not altered by supplementation with IL-1, IL-2, IL-7 or IL-15. There is no evidence that the relative frequency of regulatory T cells, either derived from the responding cells or from the host, is altered by precursor frequency. From the failure to find a limiting component for which responding cells may compete or to show changes in numbers of Tregs, we were forced to consider the possibility of a self-regulation process. This appears highly likely in view of the fact that responses are nonlinear even at very low precursor number. Initial studies indicate that even increasing precursor number from 3 to 30 can diminish the response. Thus, the current hypothesis is that there is a powerful physiologic regulatory process based on the magnitude of response within a set of responding cells that acts to feedback on cells to diminish their proliferative rate and possibly to alter their decisions regarding growth and differentiation. [unreadable] In the course of analyzing the control of FE on the part of both nave and memory cells, it was observed that the most potent stimulant of FE was the cytokine IL-1. When expansion of CD4 TCR transgenic T cells in a syngeneic host in response to antigen was measured, it was found that implanting a mini-osmotic pump that delivered 10 micrograms of IL-1 over a 7 day period caused a ten fold enhancement in FE when compared to that seen using conventional adjuvants such as LPS. This was equally true for naive and memory cells and was not mediated by other cytokines. The effect could only be partially explained by enhanced proliferation so that greater survival was also implicated. The use of recipients that were IL-1 receptor knockouts and IL-1 receptor-sufficient donors of TCR transgenic T cells showed that IL-1 could act directly on the responding T cells. Anti-IL-1 antibody diminished the adjuvant effect of LPS indicating that at least a portion of the effect of this conventional adjuvant was due to endogenous production of IL-1. Initial analysis of genes activated and suppressed in cells responding to antigen in the presence of LPS have suggested avenues for further analysis that may lead to a mechanistic understanding of the IL-1 effect. The very robust effect of IL-1 suggests it may have a role in certain immunization strategies.[unreadable] Analysis of the steady state proliferation of memory CD4 T cells reveals that it is similar in conventional and germfree mice. The similarity of the prolifertative rate in conventional and germfree mice suggests that this is not driven by intestinal microflora or conventional antigens and that it may represent self-reactivity. However, analysis of BrdU-labeling and delabeling kinetics and proliferative potential of memory and of nave CD4 T cells upon transfer to intact syngeneic recipients implies that a portion of this proliferation reflects burst-like proliferative events rather than stochastic turnover. A variety of approaches have been used to analyze the relative contribution of these two forms of proliferation to the dynamics of memory CD4 T cells. In addition to transfer experiments, that imply that 1/3 of all CD4 T cells have completed a series of up to 7 divisions within the past week, an analysis of of the behavior of in situ cells is also underway. The approach used is the identifcation of cells that have proliferated within the last week using either staining with anti-Ki67 or BrdU-labeling and measurement of the mean number of divisions of such cells using uptake of deuterated nucleotides into DNA. Model-building predicts that stochastic division should lead to a mean number of divisions of less than 1.5 whereas burst-like division should result in a mean number of divisions over 3. These experiments have been carried out and are awaiting an analysis of the incorporation of deuterium into DNA and the determination of mean numbers of divisions. [unreadable] Working with colleagues at the Oregon Regional Primate Center, we are analyzing lymphocyte dynamics in SIV-infected macaques and have obtained evidence for CD4 T cell populations that decay at very distinct rates in the period after their burst-like expansion. Laboratory of Immunology scientists and our colleagues have shown the critical role that aberrant immune activation in lentiviral infection plays in the decay of CD4 T cell number and in the degradation of immunocompetence in SIV-infected macaques and HIV-infected humans. The ongoing activation of the immune system caused directly or indirectly by infection appears to be of particular importance in the loss of potential effector cells during the chronic phase of the infection. [unreadable] Utilizing the techniques of recombineering, we have prepared indicator mice for expression of three key homeostasis-determining cytokines, IL-7, TSLP and IL-15. The IL-15 and TSLP reporter mice have shown good expression and in particular, the TSLP reporter is expressed sufficiently well to be detected in tissue samples of lungs in allergic inflammation and in skin in mice treated with Vitamin D3 analogs. [unreadable] A collaboration has been initiated with professor G. Bacherov of the Russian Academy of Sciences to use computational methods to analyze lymphocyte dynamics. With the growing numbers of tools to subdivide lymphocyte populations and to asses their function, such computational analysis will be of great value in the effort to develop realistic models of the dynamics of these cells.